Water-soluble polymers have utility as additives in many industries, such as the pulp and paper industry, the petrochemical industry, and the mining industry among others. New and improved techniques for forming these polymers are always of interest.
A common method for polymerizing water-soluble unsaturated monomers is to polymerize aqueous solutions of monomers under appropriate reaction conditions. The aqueous solution polymerization of such monomers as acrylamide, and acrylic acid among others has certain disadvantages. The process is generally a batch-type process, which can produce only dilute polymer solutions. Moreover, the batch-type polymerization process forms a hydrous polymer. This hydrous polymer usually contains 60-80% of water and is in the form of an aqueous solution, but it is a rubber-like substance which has substantially no fluidity or a viscous liquid difficult to flow because the molecular weight of the polymer is very high. Accordingly, the hydrous polymer as such is difficult to handle and uneconomical in transportation, and also it has the disadvantage that the rate of dissolution of the polymer in water when using the hydrous polymer is very low. Therefore, the aforesaid hydrous polymer is usually formed into a dried powder by removing water therefrom by some methods, and one of the methods of removing water is a method of heat-drying the hydrous polymer as obtained with hot air for example. This method is often industrially used because it is simple in principle and has many advantages with respect to production. However, in general, the dissolvability of the dried polymer obtained in water tends to be decreased in accordance with certain factors such as an increase in the monomer concentration in the polymerization step, the molecular weight of the polymer and the drying temperature among others.
When the decrease in dissovability is slight, it can be compensated for by such a means as to prolong the dissolving time when the dried polymer is used. However, when the decrease is great, the polymer is only swollen even when stirred in water for a long time, and gives a solution containing many insoluble particles. Therefore, when said solution is used for treating waste water or the like as a flocculant only a low flocculating ability is shown, and when it is used as a thickener for paper-making, fish eyes are formed on the web formed.
However, it is often desirable that the monomer concentration in the polymerization and the drying temperature are high, in respect of productivity, and that the molecular weight of the polymer is high in respect of performance. In addition, the dissolvability of the polymer must be good. Therefore strenuous efforts have been made to adjust these requirements, and as one of the means for solving this problem, attempts have been made to develop substances which improve the dissolvability of the dried acrylamide polymer. Some of these substances are those like mild chain-transfer agents which prevent a polymer having too high a molecular weight from being produced, and others and substances having an effect of preventing cross-linking in the drying step.
If it were possible to produce concentrated polymers by a solution polymerization technique which was rapid and readily produced polymers in a solid, readily dissolvable form, an advance in the art would be afforded. Also of benefit would be a method for using solution polymerization techniques to continuously produce water-soluble vinyl polymers. It is therefore an object of this invention to provide a continuous process for polymerizing water soluble vinyl monomers. A further object is to provide a method for polymerizing water-soluble vinyl monomers which produce finished polymers in a concentrated form.
Most continuous processes involve moving reactant materials horizontally as polymerization takes place. Once reagents are initially introduced and polymerization begins, the reagents and product polymer proceed in a forward direction. For example, among the continuous processes for polymerization are a technique which utilizes an endless conveyor belt as disclosed in U.S. Pat. Nos. 4,138,539; 4,893,999; 3,732,193 and 4,857,610; formation of cross-linked polymer gels by a single screw cylindrical mixer in U.S. Pat. No. 4,769,427 and polymerization within a tubular reactor as disclosed in U.S. Pat. No. 4,196,272. By their nature and configuration, opportunity for mixing to ensure complete reaction of all reagents is reduced. Therefore, there is a need for an improved continuous process which leads to the production of a more highly concentrated product of higher quality, by allowing a more complete mixing.
One method by which such objects can be achieved is by a polymerization technique which involves a continuous polymerization process performed in an extruder which allows backmixing. Such extruders, or kneaders, as they are also referred to, have been utilized for the purposes of mixing, kneading and compounding. This piece of equipment is a long rod equipped with flanges enclosed within a housing which also has flanges in the interior. The long rod is rotated or moved back and forth within the housing. A feature of this housing is that there are various temperature zones along the length of it which may be independently controlled. Reactants are placed at one end of the housing, though they may be also added at other points further down the housing. As a certain temperature range is maintained, the reactants begin to polymerize as the flow through the interior of the housing. The interaction of the long rod or screw flanges with those of the interior of the housing effects a cleaning, insuring that polymeric product does not stick to the equipment, but rather continues to move within the housing to the opposite end. Such devices have been disclosed in U.S. Pat. No. 3,601,370 and GB 1,380,149.
Polymers have been produced within extruders, though the general trend has been to utilize a twin screw extruder. A review of the use of the twin screw extruder for polymerizations is disclosed in Advances in Polymer Technology, Vol. 9, No. 4, pp. 321-330; and in Advances in Polymer Technology, Vol. 3, No. 2, pp. 99-105, and particularly Table 3 of that reference which indicates that twin screw extruders have been utilized to do free radical polymerizations which produce water-soluble polymers. A free radical polymerization method for producing homopolymers and copolymers from amido-sulfonic acid or salt containing monomers in aqueous solution in the presence of high energy mechanical mixing (defined as an extruder, wherein twin screw extruders are preferred) to produce a solid polymer is disclosed in U.S. Pat. No. 4,812,544. Continuous production of cross-linked polymers by free radical polymerization of monomers in aqueous solution in a vessel having a plurality of mutually parallel rotary stirring shafts each fitted with stirring blades is disclosed in U.S. Pat. No. 4,625,001, and the polymerization of methylmethacrylate in a counter-rotating twin screw extruder is disclosed in Polymer Process Engineering, 3 (1&2), 71-83 (1985).
Polymerizations have also been effected in single screw extruders. A process for the continuous manufacture of silicone gums form cyclopolysiloxane monomers in a single screw-kneader reactor is disclosed in GB 2,154,596. A continuous free radical polymerization of vinylic compounds such as acrylamide and acrylic acid in a wiped surface reactor is disclosed in EP 160,394. This reference discloses that vinylic compounds may be diluted by organic solvents or added neat, and specifies that the polymerization is performed such that the reactants are substantially free of oxygen, preferably in a twin screw reactor. Monomers in aqueous solution are not disclosed.
That the use of an extruder allows for enhanced mixing capabilities has been recognized. GB 1,380,149 discloses an extruder with a reciprocating mechanism which is useful for performing chemical reactions. That the mixing mechanism of an extruder comprises distributive and dispersive longitudinal mixing, during reactive extrusion is disclosed Practical Aspects of Reactive Compounding in a Continuous Kneader, Advances in Polymer Technology, Vol 11, No. 4, pp. 287-294 (1992), and in Influence of Polymer Flow Behavior on Reactive Process in Rotating-Oscillating Kneaders, presented at the SPI National Plastics Exposition Conference in 1988. Backmixing is mentioned in Reactive Extrusion of Polymers: A Review, Advances in Polymer Technology, Vol. 9, No. 4, pp. 321-330 (1989) on p. 323 as influential to residence time distribution and molecular weight distribution of polymers obtainable by free radical reaction. However, these references do not recognize that backmixing is desirable in a free radical polymerization for the more efficient production of polymers.
Backmixing has the advantage of compensating for any fluctuations in the feed systems, containing two or more independant feed streams to the reactor. This is a very important feature when the feed stream ratio is greater than 1:1. To illustrate, in Example 2 which follows, the monomer to initiator feed ratio is 80:1. Without backmixing, a plug flow situation can arise and may potentially lead to a product of inconsistent quality.
Therefore, there is still a need for a highly efficient method to continuously produce a water-soluble copolymer from monomers such as acrylamide and acrylic acid in aqueous solution by free radical reaction, which polymerization method provides good mixing, transport and control over molecular weight distribution and temperature. In a water-soluble polymerization, the ability to use a more concentrated monomer solution would be advantageous for production of dry polymers because this means that there would be less need for water which must later be removed from the polymer produced. This invention allows the use of more concentrated aqueous monomer solutions in continuous polymerization by use of a single screw reactor.
Additional advantages of the use of a single screw reactor for polymerization include: ability to add other reactants such as chain transfer agents or pH buffers at later stages of the polymerization due to the fact that for each zone of the reactor there are inlets which allow for such addition; no requirement for pre-polymerization; ability to obtain higher molecular weights, since the reaction may be run at relatively low temperature in the first and second zones, and controlled to a higher temperature at subsequent zones; and a higher free reactor volume which allows for higher residence times and the preparation of very high molecular weight solution polymers at economically feasible production rates than with conventional twin screw reactors.